Collaborative Research: Linking Deep and Shallow Crustal Processes in a Continental Arc, North Cascades, Washington

由大陆间碰撞形成的山带和在挤压过程中形成的大陆火山弧具有大规模的相似之处，包括明显增厚的地壳。在过去的十年里，许多研究人员试图使用部分熔融的中下地壳流动模型来解释碰撞山带的大规模模式，例如喜马拉雅山脉。相比之下，关于陆弧出土中的地壳流动的研究很少。华盛顿州的北喀斯喀兹是白垩纪晚期(约9000万年前)出土的弧形，其中包括曾经深达35-40公里的岩石，现在暴露在地表。这条山脉还显示了始新世(大约5000-4500万年前)邻近的非海相沉积盆地形成期间的抬升和侵蚀的证据；这些盆地充满了被侵蚀的碎片。一个跨学科的地质学家小组正在进行实地构造研究和实验室分析工作(地质年代学、岩石学、微构造分析)，以确定喀斯喀特地区变质作用、部分熔融、地壳深部岩石折返以及盆地形成和演化的时间。这些研究的目的是提供一个关于类似于玻利维亚和阿根廷现代安第斯山脉的阿尔蒂普拉诺地区的大陆弧的建造和崩溃的观点。许多研究都集中在北喀斯喀特的斯卡吉特片麻岩杂岩上。这些岩石记录了部分熔融的历史，可能发生在山脉带的折返过程中。片麻岩部分被类似于滑脱断层的低角度构造所包围。这些混合岩的流动和边界单元之间的关系以前没有被研究过，但可以提供关于地壳深部和浅部过程耦合或分离的信息。正在分析一系列构造深度的形变运动学，以确定地壳流动的方向和条件，以及流动是否发生在与北美板块边界边缘平行或倾斜的通道中。北缘盆地两侧的始新世盆地含有由走滑和正断层系统中的河流沉积的厚厚的沉积序列。与沉积岩夹层的火山凝灰岩有助于确定盆地形成的时间和沉积速率。大量始新世岩墙侵入结晶岩和盆地。岩脉测绘和测年的一个主要目标是提供山带折返和盆地形成过程中的区域变形记录。这些构造的取向可以与混合岩记录的流动方向进行比较。这项研究涉及三名研究生、本科生研究助理和暑期实习生。主要调查人员正在对北瀑布进行实地考察，所有三所大学的学生都参与其中，并在互联网上发布了一份图文并茂的、充满数据的实地考察指南，与这次旅行和研究结果有关。

Mountain belts formed by collisions between continents, and continental volcanic arcs constructed during compression, have large-scale similarities, including markedly thickened crust. In the past decade, many researchers have tried to explain the large-scale patterns of collisional mountain belts, such as the Himalayas, using models of flow of partially molten mid- to lower crust. In contrast, there have been few studies of crustal flow in exhumed continental arcs. The North Cascades, Washington, is an exhumed Late Cretaceous (circa 90 million years ago) arc, which includes rocks that were once at depths of 35-40 km and are now exposed at the surface. This mountain range also displays evidence for uplift and erosion during formation of adjacent non-marine sedimentary basins in the Eocene (approximately 50-45 million years ago); these basins are filled with the eroded debris. Field-based structural study and laboratory analytical work (geochronology, petrology, microstructural analysis) are being carried out by an interdisciplinary team of geologists to determine the timing of metamorphism, partial melting, exhumation of deep crustal rocks, and basin formation and evolution in the Cascades. These studies are aimed at providing a view of the construction and collapse of a continental arc analogous to the Altiplano region of the modern Andes in Bolivia and Argentina. Much of the research is focused on the Skagit Gneiss Complex of the North Cascades. These rocks record a history of partial melting, which probably occurred during exhumation of the mountain belt. The gneiss is bounded in part by low-angle structures that resemble detachment faults. Relationships between flow of these migmatites and bounding units have not been previously studied, but can provide information about the coupling or decoupling of deep and shallow crustal processes. Kinematics of deformation are being analyzed over a range of structural depths to determine the directions and conditions of crustal flow, and whether flow occurred in a channel parallel or oblique to the edge of the North American plate boundary. Eocene basins that flank the North Cascades contain thick sedimentary sequences deposited by rivers in a system of strike-slip and normal faults. Volcanic tuffs interlayered with the sedimentary rocks facilitate dating of the time of basin initiation, and the rates of deposition. Vast swarms of Eocene dikes intrude both crystalline rocks and basins. A major goal of the mapping and dating of the dikes is to provide a record of regional deformation during exhumation of the mountain belt and basin formation. The orientation of these structures can be compared to the flow directions recorded by the migmatites. This research involves three graduate, undergraduate research assistants, and summer interns. The principal investigators are conducting a field trip to the North Cascades involving students from all three universities, and are publishing an illustrated, data-filled field guide related to the trip and the research results on the internet.